Combo preamp/mixer/power amp/speaker cab for keyboards.
Having worked at a company that made atomic clock and GPS-based frequency standard equipment, I had access to scrap 19" rack hardware. As my brother wanted a slave amp for his bass rig to power his outboard speaker cabs, I knocked up one in a 2U rack using a 2N3055/MJ2955 transistor stage and a preamp kit board from the infamous Maplin, power supply designed by myself and cabled to the same standard of work that we used for our military clients. As a bonus, I asked the tool room manager if he could engrave the front panel with a unique design so that it was obvious this was a custom piece of hardware. The result was the DOM-200 (for my brother is Dom and it was a 200W RMS amp).



A few years later after I had moved jobs to testing mixing desks, a friend of mine's husband asked how difficult it would be to make a combo mixer amp for his gigging career as the keyboardist in a Blondie tribute band. There was nothing on the market that would do what he wanted, to be able to hook up three mono keyboards, a stereo keyboard, into a power amp which was compact enough to fit into the corner of the gig van but also have 300 Watts capability.

The parameters for enclosing the available 12" or 15" bass/mids would have made the cab woodwork rather large so I had to go with the only 10" 300W speaker I could find which also neccessitated a rather peculiar crossover frequency to the horn, the only one of which I could find was used in a JBL unit- fortunately JBL speakers were owned by the same group (at the time) that owned Allenn & Heath, so a quick word between shop floor supervisors and a scrap unit was given to me for my task.

As nearly all gigged amps get bashed about, choosing the typical chipboard seemed to be a bit short sighted so I got a 1/2" thick 10x6 foot thick sheet of marine ply and had to saw it up into the right pieces to form the cab. 120 2" screws held it together. Ripper.

Having access to certain schematics, I 'borrowed' a four channel EQ design from one of the A+H mixer input stages and designed/etched/drilled the channel and mixer PCBs for the inputs (3 mono, 1 stereo)to fit into a 3U front panel. This time because of the complexity of the milling and engraving of the front panel, I didn't think that it was in 'favour' territory to ask my friend the tool room manager to do it for free so he quoted me £85 for the work which was fine. The input, EQ and mixer stage was done, the speaker and crossover section was done, the cabinet was finished with buffalo vynide cloth, the speaker and horn was mounted- now the power amp. Design my own? Follow a reference design?

Fuck that shit, I'm not stupid. Bought a pre-made MOSFET 450W amp module from OMP and shoehorned it in. Presto, the job is done. Apologies for the crappy quality of the picture but this was from the days before digital cameras, and my pocket compact and rubbish scanner weren't up to much in those days...


( Rotating Wobbly Hat That's not a banana. THIS is a banana., Wed 17 Oct 2012, 14:33, 4 replies)

Worth it for the
scale on that Gain knob.
( chthonic, Wed 17 Oct 2012, 15:02, closed)

Haha, you said knob
though perhaps it should've said "Global Thermonuclear War".
( M3rx likes the Asian ladies, Wed 17 Oct 2012, 16:02, closed)

H-bomb test
is next to Global War, so almost in combination... if you buy one you can specify your own scale :-)
( Rotating Wobbly Hat That's not a banana. THIS is a banana., Wed 17 Oct 2012, 19:35, closed)

I am clicking this on the basis of the gain knob.
(anvil, Wed 17 Oct 2012, 18:57, closed)

I thenk yu...
I thought '1-10 is a bit passe, 1-11 is a bit too Spinal Tap, decibels are a tad too technical, so....'descriptions' are more human. The advantage of specifying your own panel engraving design :-)
( Rotating Wobbly Hat That's not a banana. THIS is a banana., Wed 17 Oct 2012, 19:08, closed)